The Geology of the Upper Jurassic - Arabian Basin
This study deals with the upper Middle to Late Jurassic generation of the basins. Greater emphasis has been given on the Hanifa Formation since it is the primary source for most of the overlying reservoirs (such as Jubaila and the Arab Members). There is a subsequent section devoted to generation of the formation. The tectonic development of the basin is dealt with briefly here. There will be a section devoted to a more description of each of the Upper Jurassic Formations; including a sample litho-stratigraphic chart for each formation. In this section a brief overview of the Upper Jurassic section is presented.

Ziegler (2001) indicated that most petroleum production from Jurassic for mostly concentrated around the intrashelf basins of Arabia (Arabian Basin, Rub' Al-Khali, and Ras al Kaimah. This is a direct result of the deposition of the upper Jurassic shelf-calcarenite Arab Formation reservoirs and the regional anhydrite Hith seal overlying the Arab Formation. The presence of mature source rock beneath the reservoirs in the Hanifa intrashelf basin controlled the distribution of the upper Jurassic oil fields in Eastern Arabia.

General Setting

The early Triassic fragmentation of the Gondwana-Laurasia super continent and the opening of the Neo-Tethys Sea (present-day east and northeast of Arabian Plate) transformed the eastern Arabian plate into a passive margin. By the early Jurassic the Arabian plate was relatively tectonically stable and was located at the Equator, so enabling the development of a wide shallow shelf on the western passive margin of the Neo-Tethys on which carbonates accumulated on the shelf and inner platform.

The climate became more humid towards the end of Early Jurassic. As a result, evaporites deposition was rare. Intrashelf depressions, such as the Lurestan Basin in the north, were created in response to tectonic differentiation and rising sea-level. By the late Middle Jurassic, the high Rimthan arch started to separate the Lurestan Basin into the Gotnia Basin in the northern present-day Arabian Gulf and the Arabian Basin in the west and south of the present-day Arabian Gulf (Husseini, 1997). The NE-SW Qatar arch separated the Arabian intrashelf basin from Rub' AlKhali intrashelf basin (also called the Southern Gulf Basin of Qatar and the United Arab Emirates) to the southeast. (Koepnick et al., 1993). The environment was progressively hot and arid, eustatic sea level was high, since there is little evidence of a clastic sediment supply, and carbonate rates of production were high, and there was an abundnce of evaporites in the late stages of the second order upper Jurassic sequence.

The major inundation of the Arabian platform was initiated in the late Callovian, which caused the deposition of the organic rich rocks that form the major source formation in the anoxic intrashelf basins of the Middle East (e.g. the Gotnia, Arabian, and Rub' AlKhali Basins).

The late Oxfordian to early Kimmeridgian Hanifa source rocks were deposited in the intrashelf basin of the differentiated Arabian Basin. These deposits were mostly low-energy, laminated, dark, organic-rich lime muds that collected under anoxic condition bottom-water conditions. To the north of this intrashelf basin, high-energy, shallow-water grainstones and evaporitic peritidal sediments accumulated across the Rimthan Arch (McGuire, 1993). To the east, the continental margin separated the platform from an open ocean in which little or no deposition took place (Murris, 1980). You can refer to Hanifa sequence stratigraphy for detailed description of the Hanifa Formation.

The Hanifa Formation is overlain by the Lower Kimmeridgian shallow-water Jubaila Formation. The Jubaila was deposited in a transitional period prior to the associated with the subsequent more regressive Arab Formation. The Jubaila Formation can be divided into three sections and they represent a shallowing upward sequence. The lower part consists of organic-rich, dark-gray, laminated, silty, lime mudstone and massive carbonate sands. These laminated deposits contain a TOC content of 0.5-3.5 wt %, indicating a source rock for both oil and gas (Alsharhan and Nairne, 1997). The middle and upper sections consist of burrowed mud, stromatoporoids and platy mudstone (Meyer et al., 1996). The Arab Formation is composed of four shallowing-upward depositional cycles each representing a transition from continuous carbonate deposition to the accumulation of nearly pure anhydrite. These units kept pace with changes in sea level until the end of Jurassic when the major evaporitic seals were deposited during a fall in sea level and the climate was predominantly very arid. Each carbonatecycle is composed of high-energy ooidal-pelletoidal grainstone containing most of the oil that accumulated in the Jurassic reservoirs (from lower to upper: Arab D, C, B, A). The largest oil accumulations occur in the lowest grainstone cycle of the Arab-D reservoir (Ayers et al., 1982).

The Arab Formation is capped by the regional Hith Anhydrite evaporite unit, which averages 167 m in thickness.The Arab Formation was deposited at the highstand stage of a lower-order sequence (supersequence) and each Arab Member (D-to-A) represents a complete increasingly regressive sequence (third-order). On the platform, carbonate deposition kept pace with and finally superseded the flooding, establishing very shallow depositional conditions over western and southern margins of the Neo-Tethys. The region become arid during the late highstand and lowstand of the subsequent sequence and as a result the Hith extensive evaporites were deposited (Murris, 1980).